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To use this chart hold it up so that the direction in which you are looking is at the bottom of the chart. The bottom edge of the chart will then represent your real horizon and the centre represents the point directly overhead. The view is correct for the UK at midnight on the 1st June, 11pm on the 15th and 10pm on the 30th. All times are given in GMT.
Transits of Venus across the face of the Sun are the rarest of astronomical phenomena, occurring in pairs eight years apart separated by more than a century. When the Sun rises on the morning of June 6, with Venus already in progress across its face, astronomers will enjoy a sight that has occurred only eight times since the invention of the telescope. Miss this one by not getting out of bed early enough and you will have to wait until 2117.
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You will need to be up early, though. The transit begins just after 11pm BST with the Sun well below the horizon from the UK. Sunrise at 4.45 will be our first chance to take a look, but less than an hour of the event will remain, so a clear eastern horizon will be essential. The main point of interest will be the egress of Venus from the disk, at about 5.37 local time.
As ever when looking at the Sun, caution will be necessary. Viewing the Sun through any form of optical aid without specialised filters is extremely dangerous. Instead, pinhole projection is probably the best way to view the transit; with a pin make a very small hole in a piece of card, and use this to project an image of the Sun’s disk. Venus should show up nicely as a dark spot; one effect to look for is the “black drop”, an apparent extension of Venus as it nears the limb of the Sun.
This effect was the bane of previous observers’ lives, as they needed accurate timings in order to use the transit to measure the size of the Solar System. The absence of the black drop was striking in most observations of the last transit, in 2004, and I’m beginning to suspect it was an effect of poor optics rather than anything more profound.
Fear of the black drop did not discourage previous generations of astronomers from going to great lengths to obtain accurate timings of the transit. The voyage of the Endeavour under the command of Captain Cook was partly a scientific expedition to view the 1769 transit from Tahiti, but the most dedicated of these early pioneers was undoubtedly French astronomer Guillaume Le Gentil.
A discoverer of galaxies and stellar clusters, Le Gentil was dispatched to view the 1761 transit from the French Indian colony of Pondicherry. Unfortunately, his journey, which took more than a year, was interrupted by the progress of the Seven Years’ War and he was forced to view the transit not from terra firma but from the rolling deck of a ship, which prevented serious scientific measurements from being made. He decided to wait for the next transit, due in 1769, and moved to Manila in order to do so. Driven from his observing site by hostile Spanish authorities, he finally reached Pondicherry and set up his observatory in preparation for the transit.
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The story does not end well. Despite beautiful clear skies for the month before the great event, Le Gentil found the sky overcast for the transit itself. His description of waking to a cloudy sky is heartbreaking: “From that moment I felt doomed, I threw myself on the bed, without being able to close my eyes ... That is the fate which often awaits astronomers.”
So affected was the observer that he was too ill to return home, and when he did set sail the voyage was blighted by dysentery, hurricanes and much else besides. Arriving home 11 years after he set out, he found he had been declared dead, his wife had remarried and his relatives had happily spent his entire fortune.
Should clouds blight the UK on the morning of the transit, I suggest avoiding existential despair by watching online. Many groups around the world will be streaming live views of the transit; for example at planethunters.org/transit. There is, of course, also more than just the transit to attend to in June. Saturn is the pick of the planets, currently in Virgo and still brighter than the constellation’s brightest star, Spica. The rings, visible in any small telescope, are tilted toward us making the Cassini Division, the major gap in the rings themselves, a tempting target. Mars and, from June 10, Mercury, are hidden among sunset twilight, whereas Jupiter appears just before dawn.
The stellar sky is dominated by the summer triangle of Deneb, Altair and Lyra in the east, and the Y shape of Boötes in the west. In between the two, looking rather more obvious on our map than it ever does in the sky, is the “keystone” of Hercules.
On this month’s map is marked the constellation’s greatest attraction, the magnificent globular cluster of M13, home to more than 300,000 stars. Just about visible with the naked eye, it appears as a fuzzy patch with binoculars, a magnificent stellar city perfectly placed for observation on warm June evenings.